A motor control device for controlling a motor monitors driving current passing through the motor and controls driving voltage so that a desired torque is output (for example, in Japanese patent laid-open publications 2004-336913, 2008-273478, and 2009-247181). In case of a tri-alternate-current brush-less motor, a motor control device performs, with a microcomputer, a control process including steps as follows. For example, the first step for obtaining three-phase alternate currents as driving current for a motor and detecting the rotation angle and the angular speed of the motor, the second step for converting, by using the rotation angle, the three-phase alternate currents into two-phase direct currents, the third step for determining two-phase direct voltages to obtain a target torque basing upon the two-phase direct currents and the angular speed, and the fourth step for converting the two-phase direct voltages into three-phase alternate voltages as driving voltage are performed. The motor control device controls the motor by applying thereto the driving voltage determined by such a control process.
Here, in the above fourth step, when the two-phase direct voltages are converted into the three-phase alternate voltages, sine and cosine of the rotation angle of the motor are used as parameters. However, from the detection of the rotation angle at the first step to the fourth step, the processing time for the second and third steps elapses. Then, since the rotor of the motor rotates in the interim, the control by the determined driving voltage lacks accuracy. Hence, a method is proposed such as to correct the rotation angle by a correction amount corresponding to the processing time for the second and third steps, and use the corrected rotation angle in the fourth step.
However, an additional step for correcting the rotation angle would be one of factors to reduce the throughput of the entire control process.